The need for articles produced from nonwoven containing nanofibers has continued to increase. The nanofibers webs are desired due to their high surface area, low pore size, and other characteristics. The nanofibers, also commonly called microfibers or very fine fibers, typically have diameters of less than 1000 nanometer or one micron. The nanofibers can be produced by a variety of methods and from a variety of materials. Although several methods have been used to make nanofiber nonwoven webs, there are drawbacks to each of the methods and producing cost effective nanofibers has been difficult.
Methods of producing nanofibers include a class of methods described by melt fibrillation. Melt fibrillation is a general class of making fibers defined in that one or more polymers are molten and extruded into many possible configurations, such as co-extrusion, homogeneous or bicomponent films or filaments, and then fibrillated or fiberized into fibers. Nonlimiting examples of melt fibrillation methods include melt blowing, melt film fibrillation, and melt fiber bursting. Methods of producing nanofibers, not from melts, are film fibrillation, electro-spinning, and solution spinning. Other methods of producing nanofibers include spinning a larger diameter bicomponent fiber in an islands-in-the-sea, segmented pie, or other configuration where the fiber is further processed after the fiber has solidified so that nanofibers result.
Melt blowing is a commonly used method of producing fine fibers and webs containing such fibers and additional particulates. The latter is generally referred to as “coforming”. Examples of coforming particulates with fine fibers have been given by U.S. Pat. Nos. 4,100,324; 4,784,892; and 4,818,464 assigned to Kimberly-Clark Corp. Typical melt-blown fiber diameters range from 1.5 to 10 microns, for example, as utilized in the above methods. Though particulates are mechanically held captive by the microfibers, there is a limitation to the functionality that particulates can provide in fine fiber webs. Therefore, it is desirable to coat the fibers with fluids or very fine particles to attain a desired functionality in the fine fiber webs.
Melt blowing can be used to make fibers with smaller diameters but with considerable changes needed to the process. Commonly, redesigned nozzles and dies are needed. Examples of these include U.S. Pat. Nos. 5,679,379 and 6,114,017 by Fabbricante et al. and U.S. Pat. Nos. 5,260,003 and 5,114,631 by Nyssen et al and U.S. Patent Application No. 2002/0117782 by Haynes et al. These methods utilize relatively high pressures, temperatures, and velocities to achieve the small fiber diameter. The above-mentioned methods have not been used to make webs with coated nanofibers.
It is an object of the present invention to provide webs containing coated nanofibers. It is also an object of the present invention to provide a method of forming a web from melt film fibrillation comprising coated fibers.